1. Field of the Invention
The present invention relates to a non-destructive inspection apparatus and method for inspecting a target structure.
2. Description of the Related Art
Conventionally, there are known non-destructive inspection methods of detecting a defect, such as a crack and a cavity, in a concrete structure, such as a tunnel and a bridge. As one of the non-destructive inspection methods, a so-called hammering test method is known.
In the hammering test method, the defect is detected from a reaction, a tone and the like brought about when a skilled inspector makes an impact on the concrete structure with a hammer. However, in this hammering test method, the reliability in detecting and diagnosing the defects, such as cracks and cavities, is low, and a difference in the diagnosis result between inspectors is large. Also, it is that a consecutively inspectable time is limited to about 30 minutes. Thus, the hammering test method is an inspection method whose load on the inspector is extremely heavy.
By the way, there are social backgrounds of rapid increase in the number of diagnosis targets from the viewpoint that concrete structures are desired to be used for a long time, and in the rapid decrease of the number of inspectors in recent years. For these reasons, new diagnosis methods have been demanded to be developed in which even a non-skilled inspector can carry out the diagnosis with a high accuracy. From this viewpoint, the development of various inspection methods, such as an infrared ray method, an X-ray method, an ultrasonic method, an electromagnetic wave method, have been tried instead of the hammering test method.
In the infrared ray method, the diagnosis is carried out based on a temperature difference between the front and periphery of the crack in the concrete structure. However, the detection of the crack is difficult in the case of an inspection target with no sunshine, such as inside a tunnel. For this reason, the method is available in tunnels only when the surface of the inspection target is heated, for example, with a halogen lamp. In addition, the method takes a long time for inspection and only a crack existing in a place less than 5 mm deep from the surface can be detected.
In the X-ray method, the treatment of the apparatus used in the inspection is not only troublesome, but also only a qualified person can carry out the inspection. In addition, a risk is involved and a cost of the inspection becomes expensive. Furthermore, the method cannot be applied to a structure, such as a tunnel, where the diagnosis must be carried out only from one side.
The ultrasonic method is a method which determines the existence or non-existence of a detect in a concrete structure by detecting the reflected wave from the defect and determines the position of the defect in accordance with a propagation time when an ultrasonic wave is irradiated to an inspection target. Typically, in the ultrasonic method, the first reflected wave from the defect is detected to measure the round propagation time from the concrete surface to the defect. However, it is difficult to detect the first reflected wave, and consequently to measure the round propagation time, since a composite wave of strongly superposed multi-reflected waves is observed by the ultrasonic sensor. For this reason, the ultrasonic diagnosis method of the concrete structure is proposed based on a multiple reflected wave motel or a standing wave model. However, in the ultrasonic diagnosis of the concrete structure based on the multiple reflected wave model or the standing wave model, an ultrasonic sensor needs to be in contact with the surface of the concrete structure. Consequently, it takes an extremely long time to carry out the inspection for a tunnel or the like, although there is no problem in its use for a case where an inspection target is small.
From the viewpoints of reducing an inspection time and improving a working efficiency of the inspection, an inspection method using an electromagnetic wave was proposed where a vehicle with an electromagnetic radar runs within a tunnel and achieves the inspection.
However, in the conventional inspection using the electromagnetic radar, diagnosis is carried out by converting the received electromagnetic wave signal into a picture of time-space intensity distribution. Thus, an excellent result is hardly obtained.